1. Technical Field of the Invention
The present invention relates to particulates of polymer resins having a high absorption capacity for water and for aqueous fluids (superabsorbents or, hereinafter, simply "SAP").
2. Description of the Prior Art
The porosity properties of the gels which result from the absorption of aqueous fluids by the superabsorbents and their mechanical moduli are of primary concern to this art. One of the principal uses of the superabsorbents is for the production of articles of hygiene, in which the absorbent function is chiefly provided by beds or sheets of individual particles or of agglomerates of such particles. The absorbency performance of the absorbent layer depends not only on the intrinsic absorptivity of the superabsorbent resin and the initial porosity of the particle beds, but also on their resistance to the blocking which occurs in use because of the loss of their absorptivity under load and because of the reduction in porosity under the influence of the pressure, or resulting from the swelling of the powder. Thus, the advantage of superabsorbents can be foreseen in which the gel particles would have an elastic modulus sufficiently high to impart thereto a sufficient resistance to distortion. Conventional superabsorbent resins are very unsatisfactory from this standpoint.
The principles of measurement of these various properties follow immediately hereafter, since it is indeed also through the observation and the comparison of their results that a quality judgment is rendered with regard to the available products and that the disadvantages are apparent which require a novel technical solution.
The particle size of the particles of superabsorbent polymer is measured on dry powder by screening.
The intrinsic capacity of the superabsorbent resin is provided by the so-called "tea-bag" test (measurement of the absorptivity and of retentivity by the "tea-bag" method, EDANA, European Association of the Nonwovens, measurement of the absorptivity and retentivity by the "tea-bag" method), by which test the water uptake is measured by weight in relation to the weight of a dry resin enclosed in a heat-sealable paper sachet, after immersion for about twenty minutes in an aqueous solution containing 0.9% of sodium chloride and draining for about ten minutes. It is expressed in g of solution containing 0.9% of NaCl in water, per g of dry SAP.
The measurement of the porosity of a bed of swollen gel is carried out in the apparatus shown in the Figure of Drawing. To accomplish this, 4 g of superabsorbent resin are swollen with 35 g of an aqueous solution of sodium chloride at a concentration of 0.9% by weight. The gel formed (G) is placed in a container (C) 52 mm in diameter and 55 mm in height. The container is next connected to the line of the reservoir (R) containing the saline solution (0.9% NaCl), optionally colored, and the quantity of solution which manages to pass through the gel is collected on a balance (B) and weighed continuously. The pressure of the liquid passing through the bed (2.5 kPa) is maintained constant by controlling the level of the feed reservoir. The mass of solution collected over 10 minutes is considered to be a measurement of the porosity of the gel.
The measurement of absorption under load of a thick bed of gel (namely, on the order of one centimeter) is an alternative form of the measurement of absorption on a thin bed (0.5 cm at the maximum), which is known to this art by the term AUL (absorption under load), and which is performed employing a device (described as apparatus for measuring the capillary suction capacity under pressure SC, in EP-Al-0258120), in which a sintered glass is placed level with and in a tank of water containing salt at a concentration of 0.9 g, on which sinter is placed a cylinder closed at its base by a sheet of nonwoven; the cylinder is filled with 0.5 g of SAP; the SAP is loaded with a weight in order to interfere with its liquid uptake. The level of the liquid is maintained constant at the height of the point of contact between the resin and the liquid. The weight and the surface of the cylinder containing the resin are selected such as to exert a pressure on the superabsorbent of 5 kPa. In the device employed in the examples given below, the weight employed was 1 kg and the diameter of the cylinder containing the superabsorbent was 5 cm, but the cylinder was charged with 1.5 g of superabsorbent powder, which is a suitable amount for the gel to have a significant thickness. The increase in the thickness of the bed of gel is a considerable obstacle to the overall swelling of the resin; conventional superabsorbents do not respond properly to this test. Thus, the absorption under load is measured in g of solution containing 0.9% of NaCl in water per g of dry SAP.